net/mlx4_en: Fix error propagation for ethtool helper function

Propagate return value of mlx4_en_ethtool_add_mac_rule_by_ipv4 in case of
failure.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>